Voltage level indicator

ABSTRACT

Integrated voltage level indicator comprising a plurality of polymer LEDs between two contact leads, in which the number of luminescing LEDs and/or the intensity of the light in each LED is determined by the external voltage level.

[0001] The invention relates to a display device comprising a pluralityof electroluminescent diodes and means for causing one or more of thediodes to luminesce in dependence upon a voltage to be supplied betweentwo connection points.

[0002] Electroluminescent (EL) or light-emitting display devices of thistype may be used in, for example displays and level indicators forchecking battery voltages, and as voltage meters in, for exampleshavers, portable telephones, for output power display in audioapplications, etc.

[0003] Such level indicators are generally known and comprise aplurality of juxtaposed light-emitting diodes (LEDs) with the voltagesupplied to the above-mentioned connection points determining the numberof LEDs which luminesces. Such a circuit comprises a plurality of LEDs,each in its own housing, which are arranged on a printed circuit boardso as to work together functionally. The supplied voltage is convertedvia an A/D converter into a digital value; the number of diodes to beenergized is determined via a second IC (or via integration in the A/Dconverter).

[0004] The costs of placing and mounting said components are generallyhigh. Moreover, the printed circuit requires extra space and covers aconsiderable part of the volume, particularly in the case of portables.

[0005] It is, inter alia, an object of the invention to obviate one ormore of the above-mentioned problems.

[0006] To this end, a display device according to the invention ischaracterized in that at least the electroluminescent diodes areintegrated in one semiconductor body.

[0007] Such an integration of the diodes reduces the costs of placementand generally reduces the number of connections to be provided.

[0008] A preferred embodiment of the display device is characterized inthat the electroluminescent diodes comprise a layer ofelectroluminescent material with a common active layer of an organicmaterial, which layer is present between a first and a second pattern ofelectrodes, at least one of the two patterns being transparent to lightto be emitted by the active layer, and the first pattern comprising amaterial which is suitable for injecting charge carriers.

[0009] Organic materials such as, for example, semiconducting organicpolymers are used for the active layer. This increases the number ofpossible materials for use in these types of display devices. The activelayer and the two electrode layers jointly constitute a plurality ofLEDs, for example, in the form of light-emitting surfaces which arearranged side by side. The operation is based on the recombinations ofelectron hole pairs which are injected into the semiconductor material(during use in the forward direction) from electrodes located on bothsides of the active layer. Due to these recombinations, energy in theform of (visible) light is released, which is a phenomenon referred toas electroluminescence. The wavelength and hence the color of theemitted light are also determined by the band gap of the (semiconductor)material.

[0010] To cause the juxtaposed light-emitting surfaces to function as alevel indicator, for example, the threshold voltage for each branchcomprising a LED is different so that, with an increasing voltage, anincreasing number of surfaces lights up. This discriminating power canbe realized in different ways.

[0011] A first embodiment of a display device according to the inventionis therefore characterized in that the electroluminescent diodes arearranged in parallel branches and are provided with mutually differentseries resistors in the parallel branches.

[0012] Since different diodes now have at least one connection incommon, the total number of connections to be provided is reduced.

[0013] The display device preferably comprises parallel resistorsarranged parallel to the electroluminescent diodes, or theelectroluminescent diodes are provided with mutually different seriesresistors between the electroluminescent diodes and a voltage divider.

[0014] In both cases, the discriminating power of the level indicator isincreased. A further improvement is obtained by using an optical filter.

[0015] Said resistors are preferably integrated in the samesemiconductor body, for example, by dimensioning the connection trackswhich are preferably made of the same transparent material (for example,indium-tin-oxide) as the transparent electrodes.

[0016] Instead of the above-described resistive discrimination,frequency discrimination is alternatively possible. A display deviceaccording to the invention, based on this principle, is characterized inthat the display device comprises a voltage-controlled oscillatorarranged between a connection point and the part of the parallelbranches which is common for the series resistors.

[0017] These and other aspects of the invention are apparent from andwill be elucidated with reference to the embodiments describedhereinafter.

[0018] In the drawings:

[0019]FIG. 1 is a diagrammatic bottom view of a part of a display deviceaccording to the invention,

[0020]FIG. 2 is a diagrammatic cross-section taken on the line II-II inFIG. 1,

[0021]FIG. 3 is an equivalent circuit diagram of the display deviceshown in FIG. 1,

[0022]FIG. 4 shows current/voltage characteristics of theelectroluminescent diodes in FIG. 3,

[0023]FIG. 5 is a diagrammatic plan view of a part of another displaydevice according to the invention,

[0024]FIG. 6 is an equivalent circuit diagram of the display deviceshown in FIG. 5,

[0025]FIGS. 7 and 8 show luminance/voltage characteristics ofelectroluminescent diodes in FIG. 5,

[0026]FIG. 9 is an equivalent circuit diagram of another display deviceaccording to the invention,

[0027]FIG. 10 shows luminance/voltage characteristics of theelectroluminescent diodes in FIG. 9, while

[0028]FIG. 11 is an equivalent circuit diagram of another display deviceaccording to the invention, and

[0029]FIG. 12 shows luminance/frequency characteristics of theelectroluminescent diodes in FIG. 11.

[0030] The Figures are diagrammatic and not to scale. Correspondingelements are generally denoted by the same reference numerals.

[0031]FIG. 1 is a bottom view and FIG. 2 is a cross-section of a part ofa display device 1 consisting, in this embodiment, of an active layer 2between two electrode layers 3, 4 of electrically conducting materials,provided on a surface 5 of a transparent substrate 6 of, for example,glass or synthetic material. The active layer 2 may comprise a polymermaterial or a low-molecular organic material (OLEDs), which material isgenerally provided by means of vapor deposition.

[0032] In this way, light-emitting diodes (LEDs) 7 are formed with theintermediate active material at the area of crossings of the electrodelayers 3, 4. During operation, the LEDs 7 emit light transverse to thesurface 5, as is diagrammatically shown by means of arrows 8. Theelectrode 3 is transparent to the emitted light in the active layer.During operation, the electrode 3 is driven in such a way that it has asufficiently positive voltage with respect to the electrode 4 for theinjection of holes into the active layer. The material of this electrode3 has a high work function and is generally constituted by a layer ofindium oxide or indium tin oxide (ITO). Particularly ITO is suitablebecause of its good electrical conductivity and high transparency.

[0033] With respect to the electrode 3, the electrode 4 serves as anegative electrode for the injection of electrons into the active layer.The material for this layer is aluminum in this example, but it may bealternatively a material having a low work function, for example,indium, calcium, barium or magnesium. If necessary, the display deviceis provided with a protective coating 9, shown by way of broken lines,on the side of the electrode layer 4. Alternatively, the layers may beprovided in the reverse order.

[0034] The electrode 4 is contacted by means of connections 10 at thelower side (light-emitting side) via contact holes 11 through the activelayer 2. For the sake of easy processing, the connections 10 arepreferably made of the same material as the electrodes 3. The material12 in the contact holes 11 may be ITO or aluminum, or any other materialused for the electrode layers 3, 4.

[0035] As already noted, light-emitting (electroluminescent) diodes(LEDs) 7, four LEDs 7 ^(a), 7 ^(b), 7 ^(c), 7 ^(d) in the embodimentshown in FIG. 1, are formed together with the intermediate activematerial 2 at the area of crossings of the electrode layers 3, 4. Theother part of the electrode layer 3 does not cross the electrode layer 4so that no light is emitted here during operation. This part isdimensioned in such a way that it comprises four series resistors 13^(a), 13 ^(b), 13 ^(c), 13 ^(d) between the LEDs 7 and a connectionstrip 14 for external connections. The series resistors 13 ^(a), 13^(b), 13 ^(c), 13 ^(d) have such a width/length ratio (w/l) that itincreases from left to right (and hence the series resistance) in FIG.1.

[0036]FIG. 3 is an equivalent circuit diagram of such a display devicewith five parallel-arranged LEDs 7 ^(a), 7 ^(b), 7 ^(c), 7 ^(d), 7 ^(e),provided with (series resistors) 13 ^(a), 13 ^(b), 13 ^(c), 13 ^(d), 13^(e), the resistance of which increases again from left to right. FIG. 5shows the current/voltage characteristics a, b, c, d, e of each branch15 ^(a), 15 ^(b), 15 ^(c), 15 ^(d), 15 ^(e) (the luminance/voltage curveis comparable therewith). From a certain threshold voltage V_(th), allof the five branches conduct. Due to the higher resistance of eachsubsequent branch, the increase of current through the respective LEDs 7^(a), 7 ^(b), 7 ^(c), 7 ^(d), 7 ^(e) is smaller at an increasing voltageV of the voltage source 16 in each subsequent branch, but the luminanceL nevertheless increases in all LEDs. As a result, all LEDs light up atthe threshold voltage V_(th), be it with a different luminance. Byproviding the display device with an optical filter 17, a part of thelight (to a luminance Lo) is attenuated and the LEDs 7 ^(a), 7 ^(b), 7^(c), 7 ^(d), 7 ^(e) light up from threshold voltages V^(′a) _(th),V^(′b) _(th) . . . V^(′c) _(th) which are different for each LED. Inthis way, the LEDs seemingly extinguish one by one when the voltage ofthe voltage source 16 decreases (for example, the battery of a shaver).

[0037]FIG. 5 is a plan view of a variant of a part of the display deviceshown in FIG. 1. The ITO track 18 between the series resistor and acontact hole 10 to the electrode layer 4 constitutes a resistor which isarranged parallel to the LED 7.

[0038]FIG. 6 is an equivalent circuit diagram of such a display devicewith five parallel-arranged LEDs 7 ^(a), 7 ^(b), 7 ^(c), 7 ^(d), 7 ^(e),provided with series resistors 13 ^(a), 13 ^(b), 13 ^(c), 13 ^(d), 13^(e) whose resistance increases again from left to right, and parallelresistors 18 ^(a), 18 ^(b), 18 ^(c), 18 ^(d), 18 ^(e). FIG. 7 shows theluminance/voltage characteristics a, b, c, d, e of each branch 15 ^(a),15 ^(b), 15 ^(c), 15 ^(d), 15 ^(e). Due to the presence of the parallelresistors 18, the LEDs 7 ^(a), 7 ^(b), 7 ^(c), 7 ^(d), 7 ^(e) light upfrom threshold voltages V^(′a) _(th), V^(′b) _(th) . . . V^(′c) _(th)which are different for each LED. Now, an optical filter is notnecessary to cause the LEDs 7 to light up separately. An optical filter17 having different optical properties is suitable for causing theluminance/voltage characteristics a, b, c, d, e of each branch 15 ^(a),15 ^(b), 15 ^(c), 15 ^(d), 15 ^(e) to converge to one and the same point(FIG. 8) so that at a higher voltage (charged battery in theabove-mentioned example) all LEDs have the same luminance. The opticalfilter may be chosen to be such that all LEDs 7 ^(a), 7 ^(b), 7 ^(c), 7^(d), 7 ^(e) have the same luminance at one and the same voltage (5 V inthis example).

[0039] The display device shown in FIGS. 3, 4 operates from a thresholdvoltage from about 2 volts, with the LEDs consecutively switching onbetween 2 volts and 2.8 volts. This “switch-on path” can be extendedand, if necessary, moved by the voltage to be measured via a voltagedivider. The voltage divider may form part of the complete displaydevice. At high battery voltages, the current through the LEDs thenremains limited, which increases the lifetime. If necessary, lowervoltages (for example, output voltages of an amplifier) may be increasedvia a DC/DC converter.

[0040]FIG. 9 shows a variant of the display device of FIG. 3, in whichresistors 19 ^(a), 19 ^(b), 19 ^(c), 19 ^(d) are present between thebranches 15, whose resistance increases again from left to right (and anextra resistor 19 ^(e)). The resistors 19 are obtained by dimensioningthe electrode layer 3. FIG. 10 shows the luminance/voltagecharacteristics a, b, c, d, e of each branch 15. Due to the presence ofthe resistors 19, the LEDs consecutively switch on between 2 volts and 6volts so that the “switch-on path” is extended.

[0041] The display device shown in FIG. 9 also has an extra branch 15^(f) with a series resistor 21 and a diode 20 having a high thresholdvoltage. This can be achieved by making use of an extra (external)diode. In this way, the voltage across the LEDs is limited toapproximately 12 volts (curve f in FIG. 10). The maximum current throughthe LEDs 7 is thereby limited in favor of the lifetime.

[0042] The LEDs 7 have an inherent capacitance 22 (FIG. 11) (naturalcapacitance). Due to the difference in resistances of the multiplierresistors (series resistors) 13 ^(a), 13 ^(b), 13 ^(c), 13 ^(d), 13^(e), each branch 15 has a different cut-off frequency. The displaydevice is now driven by means of a voltage-controlled oscillator 23whose output frequency is dependent on the voltage V_(in). FIG. 12 showshow the current and hence the luminance in the different branches 15(and hence in the LEDs 7) decreases with an increasing frequency. Eachbranch has a different cut-off frequency corresponding to a differentthreshold voltage at the input 24. With a decreasing voltage V_(in), thesupplied frequency decreases in this example, but the display device mayalso be adapted in such a way that this frequency increases.

[0043] The invention is of course not limited to the examples shown, butseveral variations are possible within the scope of the invention. Forexample, an optical filter may be used again in the display device ofFIG. 9, which simplifies the dimensioning of the resistors. Themultiplier resistors 13 may be alternatively realized between the LEDs 7and the connection 3.

[0044] In summary, the invention relates to an integrated voltage levelindicator comprising a plurality of polymer LEDs between two connectionpoints, with the number and/or intensity of the light emitted by theLEDs being determined by an electric voltage supplied to the connectionpoints.

[0045] The invention resides in each new characteristic feature and anycombination of characteristic features.

1. A display device comprising a plurality of electroluminescent diodesand means for causing one or more of the diodes to luminesce independence upon a voltage to be supplied between two connection points,characterized in that at least the electroluminescent diodes areintegrated in one semiconductor body.
 2. A display device as claimed inclaim 1, characterized in that the electroluminescent diodes comprise alayer of electroluminescent material with a common active layer of anorganic material, which layer is present between a first and a secondpattern of electrodes, at least one of the two patterns beingtransparent to light to be emitted by the active layer, and the firstpattern comprising a material which is suitable for injecting chargecarriers.
 3. A display device as claimed in claim 1, characterized inthat the electroluminescent diodes are arranged in parallel branches andare provided with mutually different series resistors in the parallelbranches.
 4. A display device as claimed in claim 3, characterized inthat said display device comprises parallel resistors arranged parallelto the electroluminescent diodes in the parallel branches.
 5. A displaydevice as claimed in claim 3, characterized in that theelectroluminescent diodes are provided with mutually different seriesresistors, and the parallel branches are arranged between a common pointand connections to a voltage divider.
 6. A display device as claimed inclaim 5, characterized in that the voltage divider comprises resistors.7. A display device as claimed in claim 3, characterized in that thedisplay device comprises a voltage-controlled oscillator arrangedbetween a connection point and the part of the parallel branches whichis common for the series resistors.
 8. A display device as claimed inclaim 3 or 6, characterized in that series resistors or resistors areintegrated in the semiconductor body.
 9. A display device as claimed inclaim 3, characterized in that the display device comprises a diodehaving a steeper current/voltage characteristic than theelectroluminescent diodes, which diode is arranged between a connectionpoint and the part of the parallel branches which is common for theseries resistors.
 10. A display device as claimed in claim 8,characterized in that the display device comprises an optical filter.